As part of micro-electronic and micro-electromechanical systems (MEMS), microscale and nanoscale materials exhibit mechanical properties and deformation mechanisms that are different from their bulk counterparts. Accurately predicting material response requires understanding the fundamental mechanisms of material deformation and fracture occurrence in microscale and nanoscale. Material properties typically cannot be extrapolated from their respective bulk values, since material behavior often is not only different in microscale, but is also significantly affected by microstructure, sample size, and/or fabrication processes, and further is very sensitive to the influences of interfaces and adjoining materials. Changes in grain size and sample texture may lead to different responses even for the same materials. Some deformation mechanisms that are unimportant at bulk scale may become dominant as the sample's volume decreases and the relative surface area increases, such as in thin films. For example, samples made by gas deposition may exhibit different characteristics from those obtained by chemical clustering. As another example, samples fabricated by mechanical attrition of metal powders may behave differently than the ones that are segmented from bulk materials.
Experiments to determine mechanical behavior and deformation mechanism at macroscale have been largely successful. Yet, as the need for using smaller-scale materials has increased with the development of microelectronics and micro-sensors, it becomes increasingly important to assess the mechanism of deformation and failure of materials at microscale and nanoscale. However, due to the limited number of testing techniques for microscale and nanoscale samples, characterizing materials at these small scales has been a challenge, and much effort has been put into developing apparatuses and methods for testing.
Various material properties tests require tensile loading of a material sample (specimen). One popular testing method is the uniaxial tensile test. This test involves gripping a specimen at opposing ends, and producing tension in the specimen along a single axis while measuring the specimen's response. In the microscale or nanoscale, however, certain challenges arise when loading specimens, such as: gripping of the specimen; aligning of the specimen in the direction of the force (to minimize likelihood of invalidation of the test caused by flexural stress on the specimen and resultant premature failure); and generating small forces (e.g., on the order of micro-Newtons) with high resolutions.
These challenges can be addressed by using a substrate layer that is usually very compliant and with known material properties along with the actual specimen to be tested. However, introduction of the substrate complicates the experimental analysis because the microscale material properties of the substrate itself may not be known accurately, and because the interface with the substrate may influence the mechanical behavior of the specimen.
For example, a method of fabricating freestanding aluminum films includes evaporating metal film on a glass slide covered with a water-soluble layer, releasing the thin film from the glass slide by immersing it in water, and gluing the film to grips of a nano-tensilometer with epoxy. However, problems of mounting the specimen and premature specimen failure invalidate a significant number of tests using this method, and experimental results from the tests have shown significant variation in measured elastic modulus and ultimate tensile strength.
Another prior fabrication technique includes sputtering metal film on glass slides and releasing the films by peeling the films off from a substrate. A motor-driven micrometer is used to produce elongation in the films, and a load cell is used to read the stress. Laser spots diffracted from the gratings on the specimen surface determine the strain with 0.002% resolution.
Yet another known method provides a piezo-actuated tensile testing apparatus using Ti—Cu—Ti multilayer films with a length of 700 microns, a width of 200 microns, and a total thickness of 1.2 μm patterned on wafers by lithography. The films are then released from the substrate by wet etching of the substrate. Such a tensile testing apparatus has been known to provide force and displacement resolutions of about 200 μN and 20 nm, respectively. Still another testing method uses piezoelectric actuators for displacement with a load cell, a laser interferometer, and a strain gauge-optical encoder assembly to measure force and displacement.
A more recent material testing method is disclosed in U.S. Pat. No. 6,817,255, issued Nov. 16, 2004 (the '255 Patent) to Hague and Saif, which is incorporated herein by reference. The '255 Patent discloses an apparatus and method for uniaxial tensile testing of a thin film material. This apparatus allows quantitative study of thin metal films down to very small thicknesses. The compact size and displacement-based measurement of example devices in the '255 Patent allows one to conduct in-situ quantitative and qualitative tensile testing in environments such as a transmission electron microscope (TEM) and a scanning electron microscope (SEM).
An example apparatus disclosed in the '255 Patent includes a testing stage (e.g., a compact MEMS-based chip) that includes a co-fabricated thin film specimen to be tested, held by at least one force sensor beam at a first longitudinal end and by a support structure at a second longitudinal end. An example support structure includes a longitudinal beam connected to the second longitudinal end and aligned with the tensile axis of the specimen, and a plurality of lateral support beams. The support beams reduce flexing of the specimen resulting from misalignment of the pulling direction with the tensile axis of the specimen, thus ensuring that a sample is subjected to uniaxial tension. Preferred embodiments of the chip include a pair of structural springs fabricated for maintaining structural integrity between the first and second ends of the chip and for addressing misalignment. Markers (e.g., displacement gauges) may be provided for measuring displacement of longitudinal ends of the specimen and deflection of the force sensor beam.
To test the thin film sample, the ends of the stage are separated (e.g., pulled) from one another by an actuator, such as a piezo actuator in SEM or a motor in TEM, which produces a tensile load on the sample. Measured displacement is used to determine material properties of the thin film specimen. For example, the force on the sample is determined from the displacement of the force sensor beams and the spring constant of the force sensor beam(s). The spring constant may be determined mathematically given dimensions and properties of the force sensor beams and/or by calibration, such as by using a nanoindenter. Sample stretching may be measured, for example, by measuring displacement of the force sensor beams and the support structure.
Another testing stage for testing thin film samples is disclosed in Han, J. and Saif, M. T. A., “In Situ microtensile stage for electromechanical characterization of nanoscale freestanding films”, Review of Scientific Instruments, Vol. 77, No. 4, pp. 45102-1-8, 2006 (“Han and Saif”), which is incorporated herein by reference. This testing stage allows more accurate calibration prior to testing (e.g., without nanoindentation after cleaving the stage) and electrical resistivity measurement of the sample in addition to stress-strain response.
An example embodiment disclosed in Han and Saif uses a testing stage co-fabricated with a thin film specimen, as with the '255 Patent. The specimen is disposed between a support structure with a longitudinal beam axially aligned with a tensile axis and a plurality of lateral beams at one end, and by one or more deformable, lateral force sensor beams with a bisecting longitudinal beam at the opposite end. To protect the metal thin film sample from possible premature failure during fabrication of the testing stage, a protecting beam is provided. The protecting beam extends parallel to the co-fabricated sample and connects the support structure to the longitudinal beam bisecting the force sensor beams.
After fabricating the sample, the protecting beam is cut using focused ion beam (FIB), to provide a displacement gauge. A laterally extending beam disposed between the support structure and the force sensor beams provides a reference displacement gauge. Tensile testing is performed similarly to that described in the '255 Patent. The displacement gauges measure displacement of the sample and the force sensor beams, and are sufficiently near the sample to allow simultaneous observation of the sample stress-strain and the displacement gauges in an observation chamber.
For calibrating the force sensor beams, Han and Saif discloses using two silicon calibrators. Both calibrators are silicon chips including an opening for connecting to a moving stage, a guide spring for improving alignment, and a leaf spring for calibration. A first calibrator, known as a master calibrator, is glued to a larger substrate that allows calibration of the first calibrator's leaf spring by a nanoindenter. This leaf spring is then urged against the second calibrator's leaf spring to calibrate it. An opening of the testing stage adjacent to the force sensor beams receives the second calibrator, and guides in the second calibrator mate with guide fits in the opening to align the calibrator within the testing stage. The second calibrator's leaf spring is urged against the force sensor beams to calibrate the force sensor beams.
In addition to tensile testing, the testing stage disclosed in Han and Saif may be used for electrical resistive testing of a sample. A metal film is deposited over the (silicon) testing stage substrate during fabrication, and metal isolations, formed by removing metal and oxide, are patterned in the testing stage to provide isolated electrodes coupling the thin film specimen to connection points. Han and Saif disclose providing a four-point resistance measurement using the isolated electrodes.
However, both the testing stage disclosed in the '255 Patent and the testing stage disclosed in Han and Saif use a co-fabricated thin film sample. This can limit the types of samples, and sample materials, that are available for testing. Further, both testing stages allow tensile testing, but not other types of material testing, such as compressive testing.